We describe a technique that maps unranked trees to arbitrary hash
codes using a bottom-up deterministic tree automaton (DTA). In
contrast to other hashing techniques based on automata, our
procedure builds a pseudo-minimal DTA for this purpose. A
pseudo-minimal automaton may be larger than the minimal one
accepting the same language but, in turn, it contains proper
elements (states or transitions which are unique) for every input
accepted by the automaton. Therefore, pseudo-minimal DTA are a
suitable structure to implement stable hashing schemes, that is,
schemes where the output for every key can be determined prior to
the automaton construction. We provide incremental procedures to
build the pseudo-minimal DTA and the mapping that associates an
integer value to every transition that will be used to compute the
hash codes. This incremental construction allows for the
incorporation of new trees and their hash codes without the need to
rebuild the whole DTA from scratch.

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